Configure OVH IP failover for FreeBSD

Summary:
An ESXi hypervisor hosted on the OVH network received as main IPv4
an IP failover (IPFO).

The gateway is the one of the hypervisor, regardless the IPFO.
As such, a static route is needed between the IPFO and the GW.

Reference: https://docs.ovh.com/fr/dedicated/network-bridging/

Test Plan: Tested on router-001

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2331